This invention relates to an X-ray source and more particularly relates to a novel wide area X-ray source.
X-ray source tubes are well known and commonly employ a tube having a cathode heated by a filament which produces an electron beam which is focused on a small area target region on an anode. X-rays are then generated at that small target region and the X-ray beam is then directed toward the region of application. Since the focused electron beam at the anode causes extreme heating, the anode is commonly rotated so that the X-ray emission region of the anode is constantly moved, thereby preventing localized overheating of the anode surface.
X-ray tubes of the above noted type have numerous failure modes. These include: burning out of the electron filament source; anode heating and pitting of the anode or target by the highly concentrated X-ray beam; plating of the anode material on the interior walls of the tube; and failure of the bearings in the high speed rotor. Moreover, the source of X-rays is essentially a poor point source since the heated target region on the anode which emits X-rays is rarely smaller than one millimeter square. In its design, a trade off is made between focal spot size, spatial resolution and ample heat capacity.
The use of a cold cathode rather than a filament heated cathode avoids the problems stated above for prior art X-ray tubes. Thus, the use of a cold cathode avoids the need for a heated filament and the cold cathode can form a relatively wide surface area source of energetic electrons. Thus, a high density spot on the anode is also avoided.
A cold cathode diode used as an X-ray source is known for use as a source of preionization energy for a discharge-excited laser in which a broad area, collimated X-ray flux pre-excites the gas of a laser tube. A device of this type is sold by Helionetics, Inc., under the name HXP-Series X-Ray Preionizer. A cold cathode diode tube X-ray source is also disclosed in European patent application publication No. 0101043, filed Aug. 8, 1983, by Helionetics, Inc. of Irvine, Calif.
Use of a cold cathode tube X-ray source in diode form, as disclosed in the above European patent application and used in the HXP-Series X-Ray Preionizer produces X-rays with a variable energy spectrum during the operation of the tube. Thus, the bremstrahlung of a given tube is related to its peak operating voltage (KVpeak). In a cold cathode tube it is known that the tube strikes at a relatively high peak voltage, and, but after the tube begins to conduct, the KVpeak reduces to a relatively low value and varies with tube current. Consequently, the bremstrahlung or spectra of the emitted X-rays changes during the tube operation. A constant bremstrahlung content, however, is necessary to obtain proper gray scale rendition when the tube is used, for example, for medical diagnostic purposes. Note that this is not significant when the output X-ray beam is used for preionization of a laser gas. However, the X-ray output of the cold cathode diode of the above European patent application cannot be used for diagnostic purpose or other purposes requiring a constant spectral distribution in the output X-ray beam.
Note that in the heated filament X-ray tube of the prior art, there is only a single KVpeak which is employed in the tube operation (unlike a cold cathode tube) so that the X-rays produced in such a tube have the requisite constant spectral distribution during the tube operation. Moreover, great pains are taken on the control systems of such tubes to insure a constant KVpeak. With a cold cathode tube, however, if the tube fires at 150 KV, it may drop to 100 KV or less during operation. The tube will conduct for only up to a maximum of about 1 microsecond but the bremstrahlung content at the 150 KV level will be present for an appreciable portion of the entire pulse period thus drastically effecting the spectral distribution of the beam during its duration. After this tube is in arc conduction, the tube voltage varies with arc current, causing further change in the bremstrahlung spectrum. For the above reasons, it has not been possible to apply cold cathode diode type tubes to the production of X-rays for diagnostic purposes or other purposes requiring a constant bremstrahlung spectrum.